1. Field of the Invention
The present invention concerns so-called "Cuk" type direct/direct or d.c./d.c. voltage converters.
2. Description of the Prior Art
The general diagram of converters of this type is illustrated in FIG. 1.
On the primary side 10 a first circuit comprising, in series, an inductance coil 11, a capacitor 12 and a transformer primary winding 13 which receives, at its terminals, the voltage V.sub.e. The capacitor 12 and the winding 13 can be short-circuited periodically under the control of a control circuit 30 which controls an active switch-over element 14 such as a transistor (generally a power MOS transistor).
On the secondary 20 side, there is a similar circuit, comprising a second series circuit with an inductance coil 21, a capacitor 22 and the secondary winding 23 of the transformer, a circuit wherein the capacitor 22 and the winding 23 can be short-circuited by a passive switch-over device, generally a diode 24 mounted as a recovery diode.
It will be noted that the windings 13 and 23 of the transformer may be virtual windings if a turns ratio of 1:1 is desired, and that there is no need for galvanic insulation. It suffices then to replace the windings with two crossed wires connecting the primary and secondary, or two non-crossed wires, thus giving, at the output, a voltage -V.sub.s which, however, makes it possible to combine the two capacitors 12 and 22 in one and the same capacitor.
Although, hereinafter, we shall consider the transformer and its windings, the present invention is not restricted to Cuk converters that effectively include a transformer but can also be applied to Cuk converters wherein the windings are virtual, all other things being equal.
A Cuk converter generally works as follows: when the transistor 14 is conductive, the input voltage V.sub.e is applied to the terminals of the inductance coil 11. This has the effect of charging this inductance coil. At the same time, this transistor in conduction connects the capacitor 12 to the terminals of the winding 13, the effect of which is to apply a negative voltage to the terminals of the transformer (the capacitor had been loaded at a voltage -V.sub.e during the preceding cycle). Simultaneously, at the secondary winding 23 of the transformer, there will be a voltage V.sub.2 proportionate to the voltage V.sub.1 applied to the primary winding 14 (or equal, if the turns ratio is 1:1), but with the it is enough to provide for low value capacitors 15 and 25 at the input and output to eliminate this single residual component.
On the contrary, in direct/direct or d.c./d.c. converters working by other principles (notably chopping systems, such as so-called fly-back or forward converters) the chopping of the input current produces a very strong ripple at both the input and output and, therefore, a high degree of interference on both the input lines and the output lines, so that it is necessary to provide for capacitors which are reservoirs of very high capacitance, as well as ballast inductance coils to prevent the lines from being excessively disturbed upline and downline whereas, on the contrary, the Cuk converter is essentially a converter without filtration.
Secondly, it is observed that the transformer always works in an a.c. mode, without any d.c. component since there is always a capacitor in series with the winding 13 (i.e. the mean voltage is zero). Thus, since the quiescent point will correspond to a null voltage, it is possible to have a very high voltage excursion before saturating the transformer so that even with a very small-sized transformer, it is always possible to be take position below this saturation limit.
Besides, the fact that, in the same way, there is an opposite sign, for the two windings of the transformer are coiled in opposite directions. The recovery diode 24 will thus get biased in the direction in which it is conductive so that the voltage -V.sub.2 will load the capacitor 22.
When the transistor 14 is no longer conductive, i.e. when the control circuit 30 puts an end to the pulse V.sub.g which ensured the conduction, the inductance coil 11 will get discharged in the capacitor 12 which will then get carried, between its terminals, to the voltage V.sub.e. On the secondary side, owing to the reversal of polarity, the diode 24 will get biased in the "off" direction, so that the capacitor 22 will get discharged in the inductance coil 21. At the secondary, winding 23 there will thus be an addition of the discharge currents of the two capacitors, one directly and the other through the transformer.
FIG. 2 shows the shape of the different signals obtained at the pace of the control pulses V.sub.g delivered by the control circuit 30 (a pulse with a duration D corresponding to the conduction state of the transistor 14).
This type of conductor has two typical advantages.
Firstly, the input and output currents, respectively marked I.sub.e and I.sub.s, have low residual ripple: this property which is inherent to the working of the converter, is obtained even when there is no filtering at all, so that alternating signal at the secondary winding 23 enables current transformers to be placed in series with the secondary winding to measure the output current, thus avoiding recourse to shunts which, in principle, produce voltage drops that increase the internal impedance of the converter.
By contrast, the Cuk type converters have the disadvantage wherein the value of the output voltage is not proportionate, as in the chopping converters, to the width of the control pulses (namely, given by a simple expression of the form (V.sub.s = k.multidot.D.multidot.V.sub.e) but are given by a non-linear expression of the following form (assuming a turns ratio of 1:1): EQU Vs = Ve .multidot. [D/(1-D)]
D being the duration or "width" of the pulse V.sub.g (expressed in terms of cyclical ratio) corresponding to the period for which the transistor 14 is put into conduction.
Besides, it is necessary to provide for a regulation of the converter so that it is possible, in controlling the delivery of the pulses in the control circuit 30, to vary the output voltage V.sub.s so as to compensate, firstly, for the variations in the output load and, secondly, for the variations in the input voltage.
Up to now, this was achieved by a regulation loop 40 (FIG. 1) receiving at the input the output voltage V.sub.s and modifying a control voltage V.sub.c of the control circuit 30 as a function of the deviation measured between this output voltage V.sub.s and a set voltage with a given reference.
The general diagram of the prior art regulation circuits and the different signals delivered are given in FIGS. 3 and 4.
A regulation loop 40 receives, firstly, the output voltage V.sub.s measured and, secondly, a reference voltage V.sub.ref and delivers, at output, a control signal V.sub.c enabling the monitoring of a slope generator formed by a current generator 31 charging a capacitor 32. The slope voltage V.sub.slope obtained is shown in FIG. 3, where it can be seen that the start of the slope is synchronized with each clock pulse H, which also determines the end of the slope. The gradient of the slope will vary as a function of the error signal V.sub.c and a comparator 33 will be used to compare the instantaneous voltage V.sub.slope of the slope with a fixed offset voltage V.sub.offset. The output signal of this comparator 33 will form the control pulse V.sub.g enabling the transistor 14 to be placed in the conductive state. It is seen that the start of the control pulse V.sub.g is determined by the instant, which is variable, when the voltage V.sub.slope reaches the level V.sub.offset while the end of the control pulse V.sub.g still corresponds to the end of the clock pulse H, namely, the instant when a change-over switch 34, controlled by the clock signal, resets the voltage at the terminals of the capacitor 32.
The result of this is that if the output voltage V.sub.s decreases (either because of an increase in the load or because of a decrease in the input voltage V.sub.e), the gradient of the slope will get rectified, taking the shape indicated by V'.sub.slope and therefore extending concommitantly with the duration of the pulse from D to D', the effect of which will be to raise the output voltage and thus compensate for the detected voltage drop.
It is seen, however, that with this prior art control and regulation circuit, it becomes inevitably necessary to find a compromise between:
firstly, the precision of regulation which will be all the greater as the gain of the regulation loop 40 is high and, PA1 secondly, the need to avoid the instabilities of the system, which is a closed loop regulation system and, hence, one that is stable only under certain conditions. This makes it necessary to provide for a sufficient gain or phase margin and, therefore, makes it necessary to reduce the precision and extend the time constant of the regulation. PA1 the gradient of each slope varies as a function of the input voltage and is independent of the output voltage; PA1 the threshold level is a function of the error signal and, PA1 the control pulses are produced between a fixed instant corresponding to the start of each slope and a variable instant corresponding to the flip-over of the comparator circuit.
In practice, it is common to use stabilization networks, placed in the loop, which reduce the passband of the system in order to prevent instabilities. However, correlatively, these networks reduce, to the same extent, the response time of the converter to a variation of the input voltage V.sub.e.
Now, in practice, it is seen that the load variations are most often relatively low (when the converters supply a set of electronic boards or analog circuits) while the biggest variations are those of the input voltage V.sub.e, which may be due to an unstable mains (a converter supplied by the mains), peaks or sudden drops of voltage caused by the upline starting up of a high power consuming apparatus, etc.
It is observed that, when there are highly unstable supply sources such as of this type (a frequent case with respect to instruments taken on board aircraft and carried far from their energy source etc.), a peak or sudden collapse of voltage at an input will always be recovered ( albeit in attenuated form) at the output because of the incapacity of the regulation circuit to take very short term variations into account, since it is necessary to provide for a minimum time constant of the loop to ensure the stability of the system.
One of the aims of the present invention is to overcome these various drawbacks in proposing a regulation and control system which, without compromising on stability, makes it possible to have a very short response time which is far shorter than the clock period.